Machine-readable codes are being used on an increasing variety of articles for the purposes of organizing tracking such articles, discouraging theft, and so forth. For example, machine-readable codes are now in common use on grocery store items, identification cards, waybills, and other applications. The parent to this application, Ser. No. 08/203,546, incorporated herein in its entirety by reference, discloses methods and apparatus for marking articles having hard, shiny, and even curved surfaces, with a machine-readable code, and systems and techniques enabling such codes to be reliably interpreted despite conditions hostile to reliable imaging. Curved, shiny surfaces, in particular, present extreme challenges with regard to machine readability. On one hand, imaging of the article surface in the vicinity of an encoded marking typically benefits from controlled illumination, but even when steps are taken to ensure that such illumination is uniformly applied, curved, specular surfaces in particular tend to produce hot spots at the detector apparatus, causing problems associated with simultaneous control over critical imaging parameters such as brightness and contrast.
A solution to this problem, disclosed in the above-referenced parent application, takes the form of a controlled environment for observing encoded marks made on such surfaces. Broadly, the approach involves indirect lighting of a surface or volume to be intentionally reflected from the article surface and onto the detector, thus enhancing visual differentiation between areas associated with machine-readable symbology, and areas of the article surrounding such symbology. Thus, rather than concentrating on uniformly illuminating the article itself, attention is instead turned to illuminating a surface reflected by the article in such a way that the reflections are of constant value and more conducive to normalization. To successfully carry out this approach, a plurality of illumination sources are supported with respect to a volume having an interior surface, causing reflections from the information-bearing surface to "see" portions of this illuminated interior volume, resulting in controlled reflections with respect to a detector mechanism. This apparatus has proved successful with regard to the decoding of machine-readable symbology associated with difficult surfaces, including hard, shiny and curved surfaces associated with metallic objects, including surgical instruments, and the like. Despite the marked improvement in the readability of such symbology, however, improvements to the basic principles introduced above have been discovered, and are disclosed and claimed herein.